Ignorant question: what happens if Mozilla or Brave or whoever says fuck that, we're not complying? What's the enforcement mechanism for non-EU-based devs publishing FOSS freely on the global internet?
The enforcement mechanism is to warn and then ban non-compliant. There are just too few playeds in the field here.
It would take only two major browser development companies to make the world 99% compliant. And the rest is statistical error no matter how safe and secure they are.
"One cannot hang a song, sure, but one can hang a singer". There are not so many places where people can get Firefox or Chromium, even fewer places where they can get source code of the named browsers.
[EDIT] grammar
The second they do that the entire internet is going to download it to see what the fuss is about.
While they could legally do that, it's going to blow up in their face if they did it. I remember the old crypto export wars in the US and OpenBSD being based in Canada so they could ship string crypto in SSH.